Axial piston engine with an inclined cylinder bore

ABSTRACT

An axial piston engine, in particular pump or motor, includes a housing, a drive shaft, and a cylinder drum. The drive shaft is mounted in the housing so as to be rotatable with respect to a first axis of rotation. A bearing body is received in the cylinder drum rotatably with respect to a second axis of rotation. The bearing body is connected to the drive shaft via a first ball joint that has a joint center lying on the first axis of rotation. The cylinder drum has at least one circular-cylindrical cylinder bore in which an elongated piston is displaceably received. A first end of the piston is connected to the drive shaft via a second ball joint. The at least one cylinder bore is arranged at an inclination to the second axis of rotation.

This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2012 222 170.0 filed on Dec. 4, 2012 in Germany, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

The disclosure relates to an axial piston engine.

An axial piston engine in a type of construction with a sloping axis is known from CH 206998 A. When this axial piston engine is operated as a pump, its drive shaft is set in rotational movement, for example by means of an electric motor, with the result that pressure fluid, in particular hydraulic oil, is conveyed. When the axial piston engine is operated as a motor, pressure fluid and pressure are delivered, with the result that the drive shaft is set in rotational movement.

The drive shaft is mounted in a housing rotatably with respect to a first axis of rotation. Arranged in the housing is a cylinder drum rotatable with respect to a second axis of rotation which intersects the first axis of rotation. The angle between the first and the second axis of rotation may be stipulated permanently or be adjustable. A separate bearing body is received in the cylinder drum and is rotatable relative to the cylinder drum with respect to the second axis of rotation. The bearing body is connected to the drive shaft via a first ball joint, the joint center of which lies on the first axis of rotation. The cylinder drum has a plurality of circular-cylindrical cylinder bores which are arranged at a constant spacing parallel to the second axis of rotation. An elongate piston is received linearly movably in each cylinder bore, a first end of the piston being connected to the drive shaft via a second ball joint.

Rotary take-up between the drive shaft and cylinder drum is brought about via the pistons. The pistons, in particular their separate piston rods, are provided with a circular-conical portion for this purpose, so that the piston rods can be tilted somewhat in relation to the assigned cylinder bore. The drive shaft can consequently be moved back and forth by a few degrees of angle in relation to the cylinder drum, until a piston rod bears indirectly against the cylinder bore, thus resulting in rotary take-up between the drive shaft and cylinder drum.

The disadvantage of this rotary take-up is that the cylinder bore is subjected to excessive wear by the take-up forces. This problem is especially serious when the cylinder drum is composed of cast iron.

The object of the disclosure is to minimize the rotary take-up forces between the drive shaft and the cylinder drum so that the wear or abrasion on the cylinder bore is reduced.

SUMMARY

According to the disclosure, this object is achieved in that the at least one cylinder bore is arranged at an inclination to the second axis of rotation. In the prior art, the cylinder bore is without exception arranged parallel to the second axis of rotation, even though axial piston engines of this type have already been known for more than 70 years. The inventors therefore had to overcome a prejudice in order to arrive at the proposed solution. It was shown, surprisingly, that even very small inclination angles lead to a considerable reduction in the rotary take-up forces. This could not have been expected by a person skilled in the art.

Advantageous developments and improvements of the disclosure are specified in the dependent claims.

The inclination angle between the second axis of rotation and the at least one cylinder bore may amount to between 0.4° and 3° and preferably to between 0.6° and 1.2°. The applicant's investigations showed that optimal contact conditions between the piston and the cylinder bore are obtained precisely at these inclination angles. In particular, the force at the contact point between the piston and the cylinder bore is oriented almost exactly tangentially. Said force has practically no radial component which causes only increased load upon the cylinder bore, without increasing the transmitted torque.

The mid-axis of the cylinder bore may intercept the second axis of rotation on that side of the cylinder drum which faces away from the drive shaft. The desired minimization of the rotary take-up forces is obtained only in this direction of inclination of the cylinder bore, but not in the opposite direction of inclination.

A plurality of cylinder bores may be provided, which are all arranged so as to be inclined to the second axis of rotation at the same inclination angle. The intention is preferably to arrange the cylinder bores rotationally symmetrically with respect to the second axis of rotation. This gives rise to especially silent running of the axial piston engine.

The piston may have a circular-conical portion which tapers toward the first end of the piston in such a way that said portion is supported linearly, at least indirectly, against the assigned cylinder bore in at least one rotary position of the drive shaft. The intention is preferably that the circular-conical portion should linearly touch the cylinder bore directly. Owing to the linear support of the circular-conical portion, the rotary take-up force is distributed over a larger area, so that surface pressure falls. The wear on the cylinder bore consequently also falls.

The piston may have a one-piece basic body which at the first end has a ball head of the second ball joint, said basic body bearing at the opposite second end directly against the assigned cylinder bore. A piston of this type is considerably more cost-effective than the known piston with a separate piston rod. The basic body is preferably provided at the second end with at least one piston ring in order to improve sealing off between the piston and cylinder bore.

The circular-conical portion may be provided on the basic body. A piston of this type is especially cost-effective.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is explained in more detail below by means of the accompanying drawings in which:

FIG. 1 illustrates a longitudinal section of an axial piston engine according to the disclosure;

FIG. 2 illustrates a longitudinal section of the cylinder drum with a piston;

FIG. 3 illustrates a sectional view along the line A-A in FIG. 2; and

FIG. 4 illustrates a sectional view corresponding to FIG. 3, the cylinder bore being oriented parallel to the second axis of rotation contrary to the disclosure.

DETAILED DESCRIPTION

FIG. 1 shows a longitudinal section of an axial piston engine 10 according to the disclosure. The axial piston engine 10 comprises a housing 20 in which a drive shaft 40 is mounted rotatably with respect to a first axis of rotation 41. For this purpose, a first rotary bearing 42 in the form of a cylindrical roller bearing and a second rotary bearing 43 in the form of a tapered roller bearing are incorporated between the drive shaft 40 and the housing 20. The drive shaft 40 projects with a drive journal 44, which may, for example, have a spline shaft profile, out of the housing 20. A radial shaft sealing ring 45 is arranged in the region of the drive journal 44 between the drive shaft 40 and the housing 20, so that no pressure fluid, in particular no hydraulic oil, can escape from the axial piston engine 10 there.

At the end lying opposite the drive journal 44, the drive shaft 40 has a flange 46. The drive shaft 40 is connected there to a plurality of pistons 80, specifically in each case by means of an assigned second ball joint 84. The second ball joints 84 are in this case arranged so as to be spaced apart from the first axis of rotation 41.

Further, a cylinder drum 60 is provided, which has a circular-cylindrical cylinder bore 62 for each piston 80. The cylinder drum 60 is preferably composed of cast iron, in particular of the material types EN-GJS-400-15, EN-GJS-400-18, EN-GJS-500-7 and EN-GJS-600-3. That end face 63 of the cylinder drum which faces away from the drive shaft 40 is curved spherically concavely, at the same time lying on a matched first end face 31 of a control disk 30. This engagement between the cylinder drum 60 and the control disk 30 forms a hydrostatic plain bearing, via which the cylinder drum 60 is mounted rotatably with respect to a second axis of rotation 61. The second axis of rotation 61 is defined by a separate bearing body 64 which engages by means of a first ball joint 65 into the flange 46 of the drive shaft 40. The joint center of the first ball joint 65 is in this case arranged at the intersection point of the first and of the second axis of rotation 41; 61. The bearing body 64 engages by means of a circular-cylindrical portion into a matched circular-cylindrical bore in the cylinder drum 60, said bore being arranged concentrically with respect to the second axis of rotation 61. The bearing body 64 is therefore displaceable with respect to the second axis of rotation and rotatable with respect to the cylinder drum 60. A spring 66, in particular a helical spring, is incorporated under prestress between the bearing body 64 and cylinder drum 60, so that the cylinder drum 60 is pressed against the control disk 30.

The second end face 32 the control disk 30 is of circular-cylindrically convex form with respect to the intersection point of the first and of the second axis of rotation 41; 61 and bears against a matched circular-cylindrical surface on the closure plate 25 of the housing 20. The second axis of rotation 61 of the cylinder drum 60 can therefore be pivoted in relation to the first axis of rotation 41. For this purpose, an adjusting device 21, which may, for example be actuated hydraulically, is provided on the housing 20. The adjusting device 21 has a movable dog 24 which engages into the control disk 30 so that said dog can displace the latter with respect to the housing 20. The movement travel of the control disk 30 is in this case limited by a first and a second limit stop 22; 23 designed in the form of screw bolts which are screwed into the housing 20. The larger the angle is between the first and the second axis of rotation 41; 61, the larger is the displacement volume, that is to say the sum of the swept volumes of all the pistons 80 of the axial piston engine 10.

The inclination of the cylinder bore 62 with respect to the second axis of rotation 61 is so small that it cannot be seen in FIG. 1.

FIG. 2 shows a longitudinal section of a cylinder drum 60 with a single piston 80. The piston 80 comprises a one-piece basic body 87 which is composed of steel. The ball head 85 of the second ball joint is provided at the first end 82 of the piston, the corresponding ball center lying on the mid-axis 81 of the piston 80. The ball head 85 has adjoining it a portion 86 which is circular-conical with respect to the mid-axis 81 and which tapers toward the first end 82. At the second end 83 lying opposite the first end 82, the piston has a spherical portion 88 with which it bears against the circular-cylindrical cylinder bore 62. The gap between the spherical portion 88 and the cylinder bore 62 is designed to be so narrow that almost no pressure fluid can pass through there. To improve sealing off, separate piston rings (not illustrated) may be attached to the basic body 87 in the region of the spherical portion 88. The circular-conical portion 86 merges, free of steps and of bends, into the spherical portion 88. The piston 80 has at the second end 83 a planar end face which is oriented perpendicularly to the mid-axis 81.

The cylinder drum 60 is provided, for example, with nine circular-cylindrical cylinder bores 62, of which only one can be seen in FIG. 2. An odd number of cylinder bores 62 is preferred. The mid-axis 68 of the cylinder bore 62 intersects the second axis of rotation 61 on that side of the cylinder drum 60 which faces away from the drive shaft, that is to say, in FIG. 2, on the right side of the cylinder drum 60. The inclination angle 67 between the mid-axis of the cylinder bore 62 and the second axis of rotation 61 preferably lies in a range of between 0.4° and 3° and amounts, for example, to 0.6° or 1.2°. The inclination angle 67 is in this case preferably selected such that the contact conditions, explained with reference to FIG. 3, between the circular-conical portion 86 of the piston 80 and the cylinder bore 62 are obtained. The inclination angle 67 is illustrated exaggerated in FIG. 2 for the sake of greater clarity.

FIG. 3 shows a sectional view along the line A-A in FIG. 2. The sectional plane is defined by a planar end face of the cylinder drum 60, said end face facing the drive shaft. This end face and therefore the sectional plane are oriented perpendicularly to the second axis of rotation 61.

The cylinder bores 62 are arranged around the second axis of rotation 61 at the same spacing with respect to the latter, the angular spacing of the cylinder bores 62 being mutually identical. On account of the inclination according to the disclosure, the cylinder bore 62 appears in FIG. 3 in the form of two circles. One circle illustrates the fully visible mouth opening of the cylinder bore 62, the other circle illustrating the bottom face of a cylinder bore 62. On account of the inclination of the cylinder bore, said bottom face is offset with respect to the mouth opening toward the second axis of rotation 61 and consequently is partially not visible and is therefore illustrated as a dashed line.

If the actual piston engine were completely rigid, only a single piston 80 would bear against the assigned cylinder bore 62 in order to bring about rotary take-up between the drive shaft and cylinder drum 60, the corresponding contact point being identified in FIG. 3 by reference numeral 90.

The connecting line 92 between the contact point 90 and the second axis of rotation 61 is depicted in FIG. 3. Further, the surface normal 91 at the contact point 90, said surface normal running through the mid-axis 81 of the piston 80, is depicted. The surface normal 91 corresponds to the direction of the force transmitted between the piston 80 and the cylinder bore 62. The angle 93 between the surface normal 91 and the connecting line 92 amounts to approximately 90°, so that said force has almost no radial force component which acts in the direction of the connecting line 92 and therefore does not contribute to the transmission of torque for the purpose of rotary take-up. Said force acts almost solely in the tangential direction.

FIG. 4 shows a sectional view corresponding to FIG. 3, the cylinder bore 62 being oriented parallel to the second axis of rotation 61 contrary to the disclosure. The angle 93 between the connecting line 92 and the surface normal amounts here to about 30°. The force acting at the contact point 90 is correspondingly higher by the factor k=1/sin 30°=2. A person skilled in the art could not have expected that the proposed small inclination of the cylinder bore 62 would bring about such a great reduction in the rotary take-up forces at the contact point 90 between the piston 80 and cylinder bore 62.

LIST OF REFERENCE SYMBOLS

-   10 Axial piston engine -   20 Housing -   21 Adjusting device -   22 First limit stop -   23 Second limit stop -   24 Dog -   25 Closure plate -   30 Control disk -   31 First end face of the control disk -   32 Second end face of the control disk -   40 Drive shaft -   41 First axis of rotation -   42 First rotary bearing -   43 Second rotary bearing -   44 Drive journal -   45 Radial shaft sealing ring -   46 Flange -   60 Cylinder drum -   61 Second axis of rotation -   62 Cylinder bore -   63 End face of the piston drum -   64 Bearing body -   65 First ball joint -   66 Spring -   67 Inclination angle of the cylinder bore -   68 Mid-axis of the cylinder bore -   80 Piston -   81 Mid-axis -   82 First end of the piston -   83 Second end of the piston -   84 Second ball joint -   85 Ball head -   86 Circular-conical portion -   87 Basic body -   88 Spherical portion -   90 Contact point between piston and cylinder bore -   91 Surface normal in the contact point -   92 Connecting line between contact point and second axis of rotation -   93 Angle between the surface normal and the connecting line 

What is claimed is:
 1. An axial piston engine, comprising: a housing; a drive shaft mounted rotatably in the housing with respect to a first axis of rotation; a cylinder drum defining at least one circular-cylindrical cylinder bore; a bearing body received rotatably in the cylinder drum with respect to a second axis of rotation, the bearing body being connected to the drive shaft via a first ball joint, the first ball joint having a joint center which lies on the first axis of rotation; and an elongated piston received displaceably in the at least one circular-cylindrical cylinder bore, the piston having a first end connected to the drive shaft via a second ball joint, wherein the at least one cylinder bore is arranged at an inclination to the second axis of rotation.
 2. The axial piston engine according to claim 1, wherein the inclination angle between the second axis of rotation and the at least one cylinder bore amounts to between 0.4° and 3°.
 3. The axial piston engine according to claim 1, wherein the mid-axis of the cylinder bore intersects the second axis of rotation on that side of the cylinder drum which faces away from the drive shaft.
 4. The axial piston engine according to claim 1, wherein a plurality of cylinder bores are all arranged so as to be inclined to the second axis of rotation at the same inclination angle.
 5. The axial piston engine according to claim 1, wherein the piston has a circular-conical portion that tapers toward the first end of the piston in such a way that the portion is linearly supported, at least indirectly, against the respective cylinder bore in at least one rotary position of the drive shaft.
 6. The axial piston engine according to claim 1, wherein the piston has a one-piece basic body that, at the first end, has a ball head of the second ball joint, the piston bearing directly at the opposite second end against the respective cylinder bore.
 7. The axial piston engine according to claim 5, wherein the piston has a one-piece basic body that, at the first end, has a ball head of the second ball joint, the piston bearing directly at the opposite second end against the respective cylinder bore, and wherein the circular-conical portion is configured on the basic body.
 8. The axial piston engine according to claim 1, wherein the axial piston engine is configured as a pump or a motor.
 9. The axial piston engine according to claim 2, wherein the inclination angle amounts to between 0.6° and 1.2°. 